n^2(n+1)(2n+1)^2(7n+1)/36.

Terms

a(0) =0a(1) =4a(2) =125a(3) =1078a(4) =5220a(5) =18150a(6) =50869a(7) =122500a(8) =263568a(9) =519840a(10) =956725a(11) =1664234a(12) =2762500a(13) =4407858a(14) =6799485a(15) =10186600a(16) =14876224a(17) =21241500a(18) =29730573a(19) =40876030a(20) =55304900a(21) =73749214a(22) =97057125a(23) =126204588a(24) =162307600a(25) =206635000a(26) =260621829